# Instrument Engineering – Locked Rotor Current of A Motor

LOCKED ROTOR CURRENT OF A MOTOR

The Locked Rotor Torque or Starting Torque is the torque the electrical motor develop when its starts at rest or zero speed. The magnitude of the Locked rotor current is determined by motor horsepower and design characteristics. Normally it will be 6 to 7 times of the Motor full Load current.

Normally Motor Name plate contains KVA code or Design Letter.

We can calculate the Locked Rotor Current Using the following Formula

Single Phase = (577 x HP x KVA/HP)/V

Three Phase= (1000  x HP x KVA/HP)/V

Find KVA/HP using the Following chart

 CODE LETTER KVA/HP RANGE CODE LETTER KVA/HP RANGE A 0.00 – 3.14 J 7.10 – 7.99 B 3.15 – 3.54 K 8.00 – 8.99 C 3.55 – 3.99 L 9.00 – 9.99 D 4.00 – 4.49 M 10.00 – 11.19 E 4.50 – 4.99 N 11.20 – 11.49 F 5.00 – 5.59 O 12.50 – 13.99 G 5.60 – 6.29 R 14.00 – 15.99 H 6.30 – 7.09

Example:

Motor Name plate indicates phase 3,300 HP,4160 Volts,KVA Code G

LRC will be

(577 X 300 X 5.60)/4160 to (577 X 300 X 6.29)/4160

=232.02 to 261.73 Amps